1. Field of the Invention
The present invention relates to a voltage controlled oscillator using a film bulk acoustic resonator and, more particularly, to a voltage controlled oscillator having a variable oscillation frequency, a communication apparatus and a frequency synthesizer using the same.
2. Description of the Related Art
In recent years, the market for a wireless communication system including a mobile phone and the like, has expanded, and at the same time, mobile communications services have been increasingly sophisticated. Moreover, it is expected that a local area network (LAN) system will rapidly become widespread in coming years. In these wireless communication systems, a radio frequency (RF) band of 2 GHz, 5 GHz, or more, is generally used.
In the RF band wireless communication systems, frequency synthesizers are used which are capable of oscillating in frequency bands required in the respective systems. The use of a quartz oscillator makes it possible to generate a highly precise reference frequency. However, a voltage controlled oscillator (VCO) is generally used in an RF band where the quartz oscillator cannot directly oscillate. Although a VCO alone cannot generate a highly precise frequency, frequency precision of a VCO or a frequency synthesizer is provided by implementing feedback control using a phase locked loop (PLL) circuit so as to generate integer or fracture times the frequency of a quartz oscillator. However, a VCO using a quartz oscillator normally provides a frequency band around 5 to 30 MHz, and even the highest frequency band of approximately 100 MHz at most.
Requirements for a VCO used in a frequency synthesizer include a wide tunability of an oscillation frequency so as to cover enough frequency range, and a low phase noise characteristic, as well as compactness and low power consumption.
A phase noise is an index to characterize dispersion of an oscillation frequency. The lower phase noise indicates that the oscillation frequency is closer to a single frequency, that is, almost ideal. The phase noise of a VCO, through frequency conversion by a frequency mixer, adversely affects the spectrum of signals in transmitting and receiving. In an orthogonal frequency division multiplex (OFDM) system, which is used for a wireless LAN system, an asymmetrical digital subscriber line (ADSL), a digital terrestrial television and the like, the lower the phase noise of a VCO, the higher the quality of signals. Accordingly, in principle, the quantity of information for transmitting and receiving can be increased.
A phase noise is caused by a thermal noise, flicker noise (1/f noise) and the like, inside an oscillator circuit. The noises emerge, as a momentary shift in the oscillation frequency, at an output node of the oscillator circuit. In order to reduce a phase noise in an oscillator, it is effective to increase the quality factor (Q value) of a resonator used in an oscillator circuit.
As a resonator exhibiting a high Q value in an RF band of GHz frequency or more, a film bulk acoustic resonator (FBAR) has been proposed recently and has collected attention. Currently, as resonators used in RF communication systems, bulk (ceramic) dielectric resonators or surface acoustic wave (SAW) devices have been used. As compared with the currently used resonators, the FBAR is suitable for miniaturization, and also for higher frequency applications. On the basis of the above reasons, a high frequency filter using an FBAR has already been commercially manufactured. Moreover, there is a proposal to use an FBAR of an aluminum nitride (AlN) as a resonator of VCO (see A. P. S. Khanna, et al., “A 2 GHz Voltage Tunable FBAR Oscillator,” IEEE MTT Symposium Digest, pp. 717–720, 2003).
Moreover, there is a proposal in which, in order to achieve a wide oscillation frequency tunability, a detection circuit is added to detect the transition of an oscillating operation from an initial state into a steady state, and a load capacitance in a resonator is connected to improve tunability of the oscillation frequency. (refer to Japanese Patent Laid-Open No. 2002-344242).
Furthermore, there is another proposal employing a wide-band frequency synthesizer in a wireless communication system, in which a wide frequency tunability is realized by means of selecting a VCO from a plurality of VCOs having different frequency bands (refer to Japanese Patent Laid-Open No. 2002-314414).
The VCO disclosed in Japanese Patent Laid-Open No. 2002-344242 is a quartz oscillator and therefore inapplicable in a GHz frequency band. In addition, the VCO requires an additional detection circuit for an oscillation signal and, therefore, is not suitable for miniaturization. Moreover, according to A. P. S. Khanna, et al., a prototype of a VCO using an AlN FBAR with an oscillation frequency of 2 GHz has achieved an extremely low phase noise (C/N). However, only a value of approximately 0.1% can be achieved for a frequency tunability. Further, in Japanese Patent Laid-Open No. 2002-314414, the frequency synthesizer is constructed by using an LC oscillator having inductors and capacitors for a resonator. Therefore, the circuitry becomes large and phase noise reduction is difficult.
As described above, currently, a VCO using a FBAR which is suitable for miniaturization and capable to oscillate a frequency range over GHz has not yet reached a point of providing a needed frequency tunablilty. Therefore, there have been few disclosed frequency synthesizers with a VCO using a FBAR.